<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>demo</title>
</head>

<body>
    <script>
        function foo(a, b, callback) {
            console.log(a + b);
            callback && callback();
        }
        foo(2, 3, calMultify);
        function calMultify() {
            console.log('回调在这一步');
        }
    </script>
</body>

</html>